Study of Free Carrier Effects for an Optical Property Modulation-Based Detector Concept for PET

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In order to overcome the intrinsic limit on the achievable coincidence time resolution (CTR) of scintillation detectors and to potentially significantly improve PET CTR, we propose to use the modulation of material’s optical properties for annihilation photon detection instead of using the scintillation mechanism. Optical property modulation mechanisms based on carrier show promise to achieve this goal. Free carrier effects depend on the change of charge carrier density in a material to modulate the material’s optical properties. This modulation can be in the optical absorption coefficient (photo-absorption) or in the index of refraction (photo-refraction). In this work, we study free carrier effects in different detector materials. The materials we experimented with include cadmium telluride (CdTe), bismuth silicon oxide (BSO), vanadium doped cadmium manganese telluride (CdMnTe:V), lead bismuth gallium (PbBiGa) heavy metal glass, and cesium lead bromide (CsPbBr 3 ). Due to different surface conditions, the crystal samples were tested with different optical setups. CdTe, BSO and PbBiGa were tested in the spatial interference setup. CdMnTe:V was tested with a Mach-Zehnder interferometer, and CsPbBr 3 was studied with a direct transmission setup. In this work a laser diode was used to provide the source of ionization-induced free carriers. To study free carrier effects, the experiments were done under zero bias voltage for the detector crystal. For comparison purposes, we also measured the modulation signal with the crystal biased at 1000 V. The CdTe crystal shows a modulation strength (unit of 1×10−4) of 7.2 at 0 V bias, and 20.8 at 1000 V bias. The BSO crystal generates a modulation strength of 4.1 at 0 V bias, and 7.5 at 1000 V bias. The PbBiGa crystal has a modulation strength of 7.2 at 0 V bias, and 6.4 at 1000 V bias. For CdMnTe:V, the modulation signal has an amplitude of 4.8 when the crystal is biased at 1000 V. For CsPbBr 3 , the modulation signal has an amplitude of 2.0 under zero bias voltage. We conclude that free carrier effects show up in CdTe, BSO, PbBiGa and CsPbBr 3 crystal samples. CdTe and PbBiGa show potential to generate the strongest free carrier induced optical property modulations.